Elastin is an amorphous protein present in the elastic fibers of tissues such as arteries, blood vessels, skin, tendons and elastic ligaments, the abdominal wall, and lungs. Unlike other fibrous tissues like collagen, elastin is unique in that it may be stretched to over 150 percent of its original length, and can rapidly return to its original size and shape. This property of elastin provides tissues that incorporate it the ability to resume their original form after stretching due to, for example, blood flow, breathing, or bending. Like collagen protein, elastin contains about 30% glycine amino acid residues and is rich in proline. Elastin differs from collagen in that it contains very little hydroxyproline or hydroxylysine. Elastin has a very high content of alanine and also contains two unique amino acids, isodesmosine and desmosine. These unique amino acids, formed after amalgamation of three or four lysines, are responsible for crosslinking of adjacent tropoelastin molecules into the resilient elastin polymer, giving it the ability to return to its original shape after stretching.
Skin aging is a complex process determined by the genetic endowment of the individual as well as by environmental factors. In developed countries, interest in cutaneous aging is in large part the result of a progressive rise in absolute number and proportion of population who are elderly. Normal or intrinsic aging induces a progressive loss of extracellular matrix (ECM), cellularity and elasticity of skin with age. Exposure of the skin to ultraviolet and visible light, numerous chemicals, as well as accumulation of calcium and certain metabolites may induce structural damage of the existing ECM, that eventually lead to loss of elasticity and formation of wrinkles as a result of local collapse of the dermal tissue supporting epidermal layers. Especially severe and permanent loss of elasticity occurs after structural damage or enzymatic degradation of the elastic fibers (i.e. mid-dermal elastolysis), in certain metabolic diseases and after menopause. Genetic diseases associated with a decrease in cutaneous elastic fibers additionally lead to loss of elasticity, lax skin and premature wrinkle formation. Costello Syndrome, Cutis Laxa and Pseudoxanthoma Elasticum, for example, lead to premature aging most noticeably characterized by wrinkling and folding of the skin in children (pre-teenage) suffering from these illnesses. Loss of elastin, in contrast to other ECM components, cannot be spontaneously replaced by fully differentiated fibroblasts residing in adult human skin. In fact, dermal fibroblasts are mostly in the quiescent state and can be only engaged in very limited tissue remodeling and repair that includes neo-synthesis of collagens, glycoproteins and proteoglycans, but exclusively lacks of elastogenesis.
While other ECM components provide the skin with mechanical strength and secure its proper hydration, the network of elastic fibers is solely responsible for skin resiliency. Elastic fibers are composed of two major components: a scaffold of 10-12 nm microfibrils made up of several distinct glycoproteins and an amorphous core, consisting of elastin. Elastin polymer is formed after enzymatic cross-linking of the multiple molecules of the 70-73 kDa precursor protein called tropoelastin. Tropoelastin (often referred as a soluble elastin) is synthesized by dermal fibroblasts and secreted in association with the 67 kDa elastin binding protein (EBP) that acts as a molecular chaperone protecting the highly hydrophobic tropoelastin molecules from intracellular self-aggregation and premature degradation and facilitating their proper assembly on the microfibrillar scaffold in the extracellular space. Tropoelastin molecules are then polymerized into the insoluble elastin via lysyl oxidase-dependent cross-linking of their lysines residues. Mature (insoluble) elastin, synthesized almost exclusively during late gestation and early childhood, is metabolically inert and remains the most durable element of extracellular matrix that may last over the entire human lifespan in undisturbed tissues. Although the primary physiological role of insoluble elastin is to serve as a structural component of elastic fibers, there is increasing evidence that both elastin and its soluble degradation products can interact with the cell surface elastin receptor and induce intracellular signals modulating cellular proliferation, migration, and synthetic abilities.